Hey there! I'm happy to help!
A hemisphere is half of a sphere. The prefix hemi- comes from Greek and means half (hemi- isn't used that much, though), so hemisphere is a half sphere.
The capacity of container is it's volume. The formula for the volume of a hemisphere is V=2/3·πr³. We get this from the sphere formula, which is V=4/3·πr³ but the hemisphere formula is half of it, giving us 2/3 instead of 4/3.
So, let's plug in our radius and find the volume of container. We will use 3.14 for pi.
V=2/3×3.14×21³
V=2/3×3.14×9261
V=19386.36
Therefore, the capacity of this container is 19386.36 centimetres cubed.
Now you can find the volume of a hemisphere! Have a wonderful day! :D